The invention relates to a method for detecting an object located in or entering an area to be monitored by a device including at least one transmitter, at least one receiver and an evaluation unit, wherein measured position values derived from reception of reflection of the transmitted beam in the monitored area are stored in the evaluation unit.
A monitoring device of this type is disclosed in German Patent No. 41 19 797 A1, wherein the objects within an area to be monitored are detected in a contactless manner. For this purpose, the area to be monitored is bounded by a reference area consisting of an arbitrary material. A device comprising a transmitter, a receiver and an evaluation unit is used to monitor the area, the so-called protective field. The device is embodied as a light scanner which includes a deflection unit along the area to be monitored for guiding the transmitted light beam. The distance of a reference surface from the device is measured with the light scanner. A measured distance value is compared with a set value in the evaluation unit. If the distance value agrees with the set value, within a tolerance range, and the light output occurring at the receiver exceeds a minimal value, a signal "protective field is unoccupied" is issued.
When an object enters the area to be monitored, the beam path between the device and the reference surface is interrupted and the distance value of the object from the device is registered in the evaluation unit. If the distance value falls outside of the tolerance range or if the light output occurring at the receiver falls below the minimum value, the signal "protective field occupied" is issued.
The advantage of this method lies in that a large degree of detection assurance can be achieved by evaluation of the measured distance values and the signal level at the receiver.
However, the method has limitations, particularly if objects are disposed very close in front of the reference surface. It is possible in this case that because of fluctuations of the measured values, the distance values for the object or the reference value registered in the device are equal or possibly are even greater for the object than for the reference surface, so that the object can no longer be detected.
Fluctuations of the measured value can be caused by faulty components, aging of the components or the like. The effect is even increased if the object and the reference surface have almost the same reflection capability, so that the received amplitudes are almost identical.
A reflection-locating system having a transmitter, a receiver and an evaluation unit is disclosed in German Patent DE 39 33 437 C2. This system scans a region with its transmission/receiving beam and determines the distance from a target object located in the region via the echo delay for the respective instantaneous beam directions. By means of a change in the beam direction, a series of distance measuring values corresponding to a change in the profile (contour) of the object is created. These values are received by and stored in a signal processor. The signal processor performs additional processing to conduct a more thorough analysis of object-specific signal contents for target identification.
A laser light sensor is disclosed in German Patent DE 36 22 421 which has a transmission light beam that is guided by optical devices within a spatial region in order to scan workpieces and the like. The transmission light beam is steered in a number of pre-programmable spatial directions, for which a predetermined receiving signal allocated to an object is stored as a reference value. To determine whether an object is present in a particular spatial region, the received signals are compared with the reference values.